There have been known a number of liquid crystal compounds including those which are called nematic liquid crystals. Although compounds or compositions mainly employed in liquid crystal display devices at present comprises these nematic liquid crystals, they have a serious disadvantage, namely, a low response speed of a several millisecond order. Thus it is considered that the enlargement of these display devices might be restricted thereby.
In order to improve this disadvantage of conventional liquid crystal display devices, N. A. Clark and S. T. Lagerwall proposed to use liquid crystals having a bistability, as described in JP-A-56-107216. (The term "JP-A" herein used means an "unexamined published Japanese patent application".) These liquid crystals having a bistability are called ferroelectric liquid crystals and attract public attention since they can give high response and memory properties. Recently, it has been frequently attempted to put these ferroelectric liquid crystals into practical use. Thus it has been urgently required to develop practically available ferroelectric liquid crystal materials.
Generally speaking, the ferroelectricity is actualized with compound having an optically active moiety in a smectic phase where the long axis of the molecule shows an orientation tilting against the normal direction of the layer made of the compound. A chiral smectic phase (hereafter referred to as S.sub.c *) is particularly advantageous from a practical viewpoint since the driving voltage thereof is relatively low.
Thus ferroelectric liquid crystals exhibit an extremely high response due to spontaneous polarization, can express a bistable state of high memory properties, has an excellent viewing angle, and are suitable for a display material of a large capacity and a large picture.
A known example of such ferroelectric liquid crystal compounds is (S)-2-methylbutyl 4-(4-decyloxybenzylideneamino)cinnomate (hereafter referred to as DOBAMBC) synthesized by R. B. Meyer et al. as described in J. Physicue, 36 L-69 (1975).
This DOBAMBC contains a Schiff base in its structure, which causes a problem in chemical stability. Therefore there has been attempted to find out ferroelectric liquid crystal compounds which are physically and chemically stable Now the main current of these studies goes toward esters such as (S)-2-metylbutyl 4-(4-n-alkoxybenzoyloxy)benzoate (hereafter referred to as CN). However, these esters would show either no S.sub.c * phase or an S.sub.c * phase, if any, within a considerably narrow temperature range. Furthermore, they are monotropic liquid crystals whose phase system achieved by heating the liquid crystals to a certain temperature differs from that achieved by cooling the same to the same temperature. Accordingly there are only a few compounds which are practically applicable, as described in Liquid Crystals and Ordered Fluids, 4 (1984).
On the other hand, there have been known sulfur-containing liquid crystal compounds such as ferroelectric liquid crystals having thioester groups, for example, alkyl thioesters as described in Liquid Crystals and Ordered Fluids, 4 (1984) and phenyl thioesters as described in JP-A-62-205056, JP-A-62-281854 and JP-A-63-27451, and ferroelectric liquid crystals containing alkyl thiophenylpyrimidine structures as described in JP-A-62-292766.
However, each of these compounds either shows a narrow S.sub.c * phase temperature range or forms monotropic liquid crystals. Thus it has not been effective to introduce a sulfur atom to the skeleton in the preparation of ferroelectric liquid crystal compounds.
Accordingly, it has been required to develop ferroelectric liquid crystals which have a wide S.sub.c * phase temperature range, compared with ester-type ferroelectric liquid crystals or known sulfur-containing ones, and are not monotropic.